In transmitter/receiver circuits, local oscillators are used to produce a reference frequency. In receiver circuits, this reference frequency can be supplied to a mixer stage in which the received signal is mixed directly or via an intermediate frequency from the carrier frequency to baseband. Any discrepancy between the carrier frequency for the received signal and the receiver's reference frequency supplied to the mixer results in transmission errors in the receiver. Such a discrepancy can be brought about on account of production tolerances, temperature and supply voltage fluctuations in the transmitter and/or receiver and resultant fluctuations in the carrier frequency and/or reference frequency or by the Doppler effect as a result of a relative movement between base station and mobile station.
To counteract transmission errors caused by frequency offset, the transmitter/receiver circuits use, by way of example, expensive, high-quality oscillators which produce a very stable, i.e. fluctuation-free and precise, reference frequency. It is likewise possible to use voltage-fluctuation and temperature-compensated oscillators to reduce the reference frequency's dependency on voltage fluctuations and temperature. In addition, “automatic frequency correction control loops” (AFC loops) are frequently used for precisely setting the local reference frequency. In an AFC control loop, the frequency offset is estimated and the estimate is used to produce a control voltage that is supplied to a voltage-controlled oscillator. The latter's output frequency is used as the input frequency for a PLL control loop. Upstream of the voltage-controlled oscillator, a low-pass filter is used for low-pass filtering of the estimate signal converted into the control voltage. To estimate the frequency offset, a pilot signal having a known content is used in the receiver, for example. If the data rate of the known signal is significantly higher than the possible rate of change of this frequency offset, then it makes sense for the latter not to be corrected until after the estimate has been low-pass filtered. This is frequently indispensable, both in order to improve the quality of the estimates (smaller variance) and in order to prevent unwanted creation of dynamics in the control process, such as oscillation. The low-pass filtering is a crucial drawback for rapid or even abrupt frequency changes.
Such an abrupt change in the apparent frequency of the transmitter occurs, for example in the 3GPP/UMTS/FDD mobile radio system, if the mobile station's reception is changed over from one base station to another base station for particular periods of time. In the “interfrequency compressed mode” operating situation, the original base station's reception is interrupted and is switched to another base station with another carrier frequency for measurement purposes. The switching time provided in the UMTS standard is extremely short in this case.
Although the UMTS standard places an extremely high demand on the base station's carrier frequency precision, apparent discrepancies of several kHz among the base stations may arise, for example on account of Doppler effects. Additional discrepancies may arise on account of switching behaviour and switching delay, as a result of needing to changeover to a different carrier frequency in the receiver. Discrepancies are also possible if the frequency produced in the oscillator or in the downstream PLL control loop for deriving the pattern frequencies differs from the one that is set, and the difference is dependent on the frequency that is set. An example of such a discrepancy would be an unknown nonlinearity in the oscillator's drive characteristic.
A further scenario for wanted rapid changes in the carrier frequency with exact timing is the initial acquisition after turning on the mobile radio. It must be assumed that the oscillator's frequency offset is next to the actual carrier frequency to such an extent that no further signal can be received or found. In this case, a signal search on a plurality of slightly offset frequencies is desirable. In this context, the changeover operation between the individual frequencies should take place as quickly and exactly as possible. Both are impaired by a low-pass filter in the control loop.
Current solutions involve no direct fast correction of the centre frequency in combination with driving the oscillator. By way of example, for the fast change between the base stations in the aforementioned “interfrequency compressed mode” the AFC control algorithm for the oscillator is stopped in the measurement gap for the second base station, and the last value in the control chain is maintained without any subsequent correction at another point. As soon as reception is switched back to the original base station again, the control algorithm is continued at the old point. In this case, losses of reception quality in the measurement gap are therefore accepted.
Another option is simply to allow the AFC control loop to continue. If the control loop's reaction is fast enough to react within the gap, impairment of the reception quality in the transitional ranges is accepted. This is shown in FIG. 1.
Changing over the search frequency for the initial acquisition likewise accepts a longer reaction time for the normal control loop.